In my prior patent applications, a unique, heat treating furnance is disclosed. The furnance uses a thin-walled, cylindrically shaped, longitudinally extending imperforate shell member disposed within a chamber or an enclosure formed in the insulated casing of a standard heat treat furnance. Heretofore, that chamber or enclosure was the heat treat chamber. By placing the work within the shell or interposing the shell member between the work and the furnance chamber a number of advantages are obtained over conventional heat treat furnaces. One of the principal benifits of such a furnance arrangement is that the shell can be pressurized and operated as a standard atmosphere furnace or a vacuum can be drawn within the shell and the furnace simply switched in operation to that of a vacuum furnance. The manufacturing cost of the furnace is about equal to or slightly in excess of the cost of a standard atmosphere furnance. The furnance casing is similar to and thus costs the same as or slightly less than that of the standard furnance while the cost of the shell member is believed to be slightly in excess of the radiant burner tubes now used in standard furnances. The costs are believed less than that of a vacuum furnance since the furnance chamber need not be vacuum welded with a surrounding water jacket throughout.
My prior patent applications incorporated by reference disclosed heating and cooling arrangements for both the outside shell surface and the inside shell surface which individually and collectively materially enhance the heat treating processing times whether the furnace be used either as a standard atmosphere furnace or as a vacuum furnace. Another material advantage residing in the furnace disclosed is the fact that gas burners can be employed to directly fire their products of combustion into the furnance chamber to heat the exterior surface of the shell and that the use of gas burners for vacuum heat treating is thus possible.
In considering various factors influencing the design of such a furnace, it is obvious that the imperforate shell member must be rather thin if the shell is to effectively function as a heat transfer exchange mechanism. Also, the shell diameter becomes large if the shell member is to hold commercial batches of workpieces typically loaded or placed into baskets or trays with load weights in excess of 1,000 pounds and a typical load volume of 24.times.36.times.20 inches. Finally, the heat treat process require high temperatures. The maximum temperature is typically above the austenitizing temperature of 1625.degree. for annealing, normalizing and heating for hardening. Carburizing takes place at even higher temperatures and heat treating of tool steels at higher temperatures yet. The thermal expansion of the shell member at such temperatures is significant, typically expanding a 40 inch diameter shell to well over 41 inches and even distorting the cylindrical shape of the shell itself.
The furnance environment requires that the furnance casing and the loading door of the furnance be cooled or cool enough to touch. Conventional sealing arrangements, at least for the front face of the furnance, use water passages in the door and the frame of the furnance casing to establish two cold surfaces which are then sealed by a low temperature elastomer seal. If this approach is tried for the shell member in the furnance disclosed herein, the heat in shell wall will come into almost instantaneous contact with a cold, water cooled surface. The temperature will rapididly drop over a short distance causing a thermal shock which will rupture the shell. Other older conventional sealing arrangement such as a fiber seal or, conceptually, a sand seal are not adequate because of the inherent leakage present in such seal arrangements which prevent a vacuum from being drawn within the shell.
The furnace of the present invention and as noted in my prior application is not entirely dissimilar, from a conceptual standpoint, than that of coil annealing covers used for some time in the steel mill box annealing processes for annealing coiled strips of steel. However, the box annealing processes used removable stand covers and removable coil covers which are thick-walled massive objects slowly heated at relatively low temperatures in a time consuming process. Importantly, the covers are sealed at their base usually by a sand seal or a loose fiber seal which inherently leak and, in fact, require a positive pressure within the coil cover to prevent leakage of the outside atmosphere into the protective annealing atmosphere within the cover. Nevertheles, the positive pressure within the cover occasionally ruins the integrity of the seal. However, leakage from the cover to the stand is not necessarily fatal to the steel mill annealing process because the stand itself is sealed.
Also bearing some resemblance to the recent invention and within the heat treat furnance art are muffle furnaces where a thick walled pipe member is structurally anchored at both of its ends to the furnace casing, thus defining a space between the pipe member and furnace casing used to heat the pipe member and the work placed therein. While such furnaces are suitable for certain applications involving continuous furnaces or furnace zones used in continuous furnaces, they are not widely used as single chamber batch type furnaces because of, among other things, the excessive processing times to heat and cool the work vis-a-vis the relatively thick walls of the muffle and the inability to use elastomer seals to efficiently seal the opening.